A communication system comprises a packet stream transforming unit, a mapping unit and a transmission unit. The packet stream transforming unit is configured to receive a 4-bit packet stream and transform the 4-bit packet stream into a 6-bit packet stream. The mapping unit is configured to map the 6-bit packet stream into multiple ternary bit streams, and the mapping unit maps at least one idle symbol into the ternary bit streams according to at least one particular bit of the at least one idle symbol of the 6-bit packet stream. The transmission unit is configured to transmit the ternary bit streams to a remote communication device through a cable.